Remove references to `Fleeting/` from active code and skills

[toejough]

Observed

The agent-memory vault tier-collapsed Fleeting/ (vault is now single-tier: MOCs/ + Permanent/, day-grained recency). Active engram code and the recall skill still reference Fleeting/ as if it exists, and engram quick actively requires it.

Concrete refs (this repo, branch opencode-plugin):

• skills/recall/SKILL.md — lines 30, 36, 115, 157 describe Fleeting/ as part of vault structure, instruct surfacing fleetings as a separate section, and reference it in the fallback path.
• internal/cli/quick.go:27 — fleetingSubdir = "Fleeting"; requireVaultDirs at line 47 fails if <vault>/Fleeting is missing.
• internal/cli/vault_fs.go:13 — comment names Fleeting/ as expected subdir.
• internal/vaultgraph/scanner.go:27,29,70 — scans Fleeting/ alongside MOCs/ and Permanent/.
• internal/cli/quick_test.go, internal/cli/export_test.go, internal/vaultgraph/scanner_test.go, internal/vaultgraph/vaultgraph_test.go — test fixtures and assertions reference Fleeting/.

(skills/learn/SKILL.md:22 already says "No Fleeting/ directory" — that one is correct.)

Repro

cd /Users/joe/repos/personal/engram-worktrees/opencode-plugin
grep -rn "Fleeting" --include="*.md" --include="*.go" \
  skills/ internal/ cmd/ 2>&1 | grep -v "^./docs/\|^./archive/\|^./playground/"

Also: pointing engram quick at the current agent-memory vault (which has no Fleeting/) errors out via requireVaultDirs — quick: vault Fleeting directory not accessible.

Expected

• skills/recall/SKILL.md: drop the Fleeting/ line from the vault-structure block; remove the separate "From recent fleetings" output section (or restate as recent-by-date within the single tier); update the missing-binary fallback to list only MOCs/ and Permanent/.
• internal/cli/quick.go: stop requiring/writing to Fleeting/. Either retire engram quick (covered by engram learn post-collapse) or rewrite it to target the single-tier vault. Decide before fixing.
• internal/cli/vault_fs.go, internal/vaultgraph/scanner.go: drop Fleeting/ from the scan set.
• Tests: update fixtures and length assertions accordingly (see Replace ANTHROPIC_API_KEY with OAuth token from Claude Code Keychain #19-style registry/length-assertion sweep — gomega truncates the second failure, so grep length assertions before editing).
• Quality gates: targ check-full clean.

Context

• Vault tier-collapse is documented in agent-memory permanents 4k.2026-05-11.vault-filename-format-current and 8i1.2026-05-11.tier-collapse-removes-tradeoff. The cleanup of consumers (this repo) wasn't propagated.
• Branch: opencode-plugin. Modified file in flight: docs/issue-612-plan.md — unrelated.
• Related: closed issues engram learn: validate --delete-fleeting path is a .md note in <vault>/Fleeting/ #608 (validation of --delete-fleeting path), agent-memory vault: sweep-and-prune permanents referencing obsolete engram learn semantics #616 (vault sweep for obsolete engram learn semantics) — both upstream of this cleanup.

Affected files

• skills/recall/SKILL.md
• internal/cli/quick.go, internal/cli/quick_test.go, internal/cli/vault_fs.go, internal/cli/export_test.go
• internal/vaultgraph/scanner.go, internal/vaultgraph/scanner_test.go, internal/vaultgraph/vaultgraph_test.go

Open question for refinement

Is engram quick still a needed surface post-collapse, or has engram learn absorbed it? The answer changes scope from "rewrite quick" to "delete quick + tests".